Building GKE Public cluster with existing Cluster CIDR Range

Encounter the error when building the GKE cluster via terraform, i.e. #googleapi: Error 400: Cluster CIDR range is greater than maximum (24 > 21)., badRequest

Just to add on, the cluster CIDR range that defined is the existing subnetwork that have created in the VPC network, which is

Understand from the GCP console portal, there is an option for choosing the existing subnetwork for that, and /24 is working fine.

Not too sure how terraform works on this.